The present invention relates to pumps for fluids, and in particular to an improved turbine pump which develops an essentially constant fluid pressure for various flow rates of the fluid.
Industrial spray coating systems for applying coatings of paint or other material to ware often use a central supply system for providing the material to a plurality of coating stations. A reservoir of the material is maintained in a tank or other relatively large container, and is pumped through supply lines to various stations whereat coating equipment, such as spray guns, is connected with the material in the lines.
Where the material is of a type which settles upon standing, such as paint, the system preferably is of a circulating type which maintains the paint in motion. With such systems paint is pumped from the bottom of the tank through a line extended past all of the spraying stations, and then is returned to the top of the tank. A mixer within the tank agitates the paint therein, and couplings in the line at the stations allow spray paint equipment to be connected therewith.
For improved coating of ware the paint ideally should be provided to the equipment at a predetermined and essentially constant pressure. Accordingly, pumps for such systems advantageously should be capable of developing a preselected and constant fluid outlet pressure. Further, since system load or paint volume requirements normally vary widely from zero in the event that no spray paint equipment is operated to a maximum where all of the equipment simultaneously is operated, the pumps also should be capable of maintaining the predetermined pressure over a wide range of flow rates.
Conventionally, pumps for such systems are of the turbine type and consist of one or more pumping stages. The outlet pressure from such pumps decreases with increasing volumetric output or flow rate therefrom, and if the pump is selected to develop the predetermined pressure under minimum flow rate conditions, then at maximum flow the pressure becomes unsatisfactorily low. In the alternative, if the pump is selected to develop the predetermined pressure at maximum flow, than at minimum flow the pressure becomes prohibitively high which requires some means, such as by-pass valves, for depleting excessive pressure.
Prior attempts to overcome the problems of fluctuating pressures with varying flow rates include selecting pumps which develop the predetermined pressure at average or mid-range flow rates. While such a compromise generally minimizes maximum pressure excursions experienced, under no load conditions the pressure may nevertheless become undesirably high and under maximum load conditions undesirably low.
Another approach to the problem of fluctuating pressures is to pump the paint at the predetermined pressure and at a flow rate which is quite high compared with maximum load requirements. Thus, for minimum or maximum loads the total flow rate from the pump, and therefore the pressure developed thereby, remains generally constant. Unfortunately, this technique has certain disadvantages, one being that a large flow rate at a given pressure requires larger and/or more pumping stages, as well as an increased power input to the pump. Of even greater concern is the frictional heating experienced by the paint when pumped through the system at a high rate of flow. Heating deteriorates the quality of the paint, and may result in color changes thereof.
To avoid undue heating of the paint it has been found that the flow rate should be no more than twice the maximum spray station requirements. That is not a large rate in proportion to maximum requirements, and therefore conventional pumps do not develop an essentially constant pressure in supplying various station requirements.